The impact of storage of potable water in open reservoirs was assessed
by examining inlet and effluent water samples from six open finished
water reservoirs used by four New Jersey utilities. Water quality para
meters investigated included Giardia cysts, Cryptosporidium oocysts, t
otal and fecal coliforms, bacteriophage, heterotrophic plate count bac
teria, turbidity, particle counts, chlorine residuals and other parame
ters. Fifteen percent of inlet samples and 25 percent of effluent samp
les contained the organisms. When data for cysts and oocysts were comb
ined, the difference in concentrations between the inlet and effluent
was statistically significant, even when results were adjusted for ana
lytical recovery efficiency. Although the concentration of protozoa in
creased following open reservoir storage, the analytical method used d
oes not permit assessment of the organisms' public health significance
. Nearly all of the cysts and oocysts were empty or contained undiscer
nible internal structures, suggesting the health risk is low. The impl
ication of these findings for watershed control programs is discussed.